Biometric, optical and physical changes in the isolated human crystalline lens with age in relation to presbyopia.
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The biometric, optical and physical properties of 19 pairs of isolated human eye-bank lenses ranging in age from 5 to 96 years were compared. Lens focal length and spherical aberration were measured using a scanning laser apparatus, lens thickness and the lens surface curvatures were measured by digitizing the lens profiles and equivalent refractive indices were calculated for each lens using this data. The second lens from each donor was used to measure resistance to physical deformation by providing a compressive force to the lens. The lens capsule was then removed from each lens and each measurement was repeated to ascertain what role the capsule plays in determining these optical and physical characteristics. Age dependent changes in lens focal length, lens surface curvatures and lens resistance to physical deformation are described. Isolated lens focal length was found to be significantly linearly correlated with both the anterior and posterior surface curvatures. No age dependent change in equivalent refractive index of the isolated lens was found. Although decapsulating human lenses causes similar changes in focal length to that which we have shown to occur when human lenses are mechanically stretched into an unaccommodated state, the effects are due to nonsystematic changes in lens curvatures. These studies reinforce the conclusion that lens hardening must be considered as an important factor in the development of presbyopia, that age changes in the human lens are not limited to the loss of accommodation that characterizes presbyopia but that the lens optical and physical properties change substantially with age in a complex manner. (+info)
Why we need reading-glasses before a zimmer-frame.
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Accommodative loss represents the fastest human biological decrement. Why? To examine this, the shape of the eye-lens during accommodation or its relaxation is analysed with special reference to age-related changes. The capsule is viewed as a force transmitter: the distribution of forces along centripetal capsular arcs and the resulting stresses and strains are calculated. The effect of zonular 'shifts' is considered. Age-related accommodative loss can be modelled successfully if changes in mechanical properties are linked to those in lenticular and capsular shape. (+info)
A physiological model to measure effects of age on lenticular accommodation and spherical aberration in chickens.
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PURPOSE: To assess physiological accommodative function of intact intraocular lenses, as measured by focal length changes, in chickens of various ages. METHODS: Eyes of white leghorn chickens, aged 0 days (hatchlings), 7 days, 14 days, 6 weeks, 1 year, and 2 years, were enucleated and the backs of the globes, except for the ciliary nerve and ganglion, were removed. The ciliary nerve and ganglion were suctioned into the tip of a suction electrode, and lenses were optically scanned before, during, and after accommodation. Accommodation was elicited by delivery of 30-Hz electrical pulses. RESULTS: For all age groups, lenticular focal lengths for stimulated eyes were significantly shorter than for relaxed eyes, indicating that accommodation had been induced. Lenticular accommodative amplitudes decreased significantly with age, with reduced function occurring between 0 and 7 days and again between 14 days and 1 year. Although accommodation was associated with an increased spherical aberration in all age groups, the increase was significant only in some age groups. CONCLUSIONS: Results showing reduction of lenticular accommodation concomitant with increasing age suggest that in spite of major differences in anatomy and physiology of the accommodative mechanism, chickens may become presbyopic. Results showing high amounts of nonmonotonic spherical aberration in hatchling lenses suggest inherently poor optics in these birds. (+info)
Age-related changes in refractive index distribution and power of the human lens as measured by magnetic resonance micro-imaging in vitro.
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We report a new technique for non-invasively mapping the refractive index distribution through the eye lens using magnetic resonance micro-imaging. The technique is applied to map the refractive index distribution throughout the sagittal plane of 18 human eye lenses ranging in age from 14 to 82 years in vitro. The results are compared with standard models for the human eye lens. They confirm that the refractive index distribution, when plotted as a function of normalised lens radius, is a function of lens age and differs both between the equatorial and axial directions and between the anterior and posterior halves of the optical axis. The refractive index of the lens nucleus exhibits a significant reduction with age amounting to 3.4+/-0.6 x 10(-4) years(-1). The contribution of the gradient index (GRIN) to the lens power decreases by 0.286+/-0.067 D/year, accounting almost entirely for the estimated overall change in lens power with age for these lenses, which were probably in their most accommodated state. The results provide experimental verification of hypothesised changes in the GRIN that have previously been invoked as contributing to presbyopia and support the hypothesis that changes in the GRIN are sufficient to offset effects of increasing curvature of human lenses with age in their unaccommodated state. (+info)
Numerical modelling of the accommodating lens.
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Data on geometric and material properties of the human lens derived from various published sources are used to construct axisymmetric, large displacement, finite element models of the accommodating lens of subjects aged 11, 29 and 45 years. The nucleus, cortex, capsule and zonule are modelled as linearly elastic materials. The numerical model of the 45-year lens is found to be significantly less effective in accommodating than the 29-year lens, suggesting that the modelling procedure is capable of capturing at least some of the features of presbyopia. The model of the 11-year lens shows some anomalous behaviour, and reasons for this are explored. (+info)
Polymer refilling of presbyopic human lenses in vitro restores the ability to undergo accommodative changes.
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PURPOSE: Because presbyopia is thought to be accompanied by increased lens sclerosis this study was conducted to investigate whether refilling the capsule of the presbyopic human lens with a soft polymer would restore the ability of the lens to undergo accommodative changes. METHODS: Accommodative forces were applied to natural and refilled lenses by circumferential stretching through the ciliary body and zonular complex. Nine natural lenses and 10 refilled lenses from donors ranging in age from 17 to 60 years were studied. Two refill polymers with a different Young's modulus were used. The lens power was measured by a scanning laser ray-tracing technique, and lens diameter and lens thickness were measured simultaneously while the tension on the zonules was increased stepwise by outward pull on the ciliary body. RESULTS: In the natural lenses the older lenses were not able to undergo power changes with stretching of the ciliary body, whereas in the refilled lenses, all lenses showed power changes comparable to young, natural lenses. The refilled human lenses had a higher lens power than the age-matched natural lenses. The Young's modulus of the polymers influenced the lens power change when measured with the ciliary body diameter increased by 4 mm. CONCLUSIONS: Refilling presbyopic lenses with a soft polymer enabled restoration of lens power changes with mechanical stretching. Because sclerosis of the lens is an important factor in human presbyopia, refilling the lens during lens surgery for cataract could enable restoration of clear vision and accommodation in human presbyopia. (+info)
Changes in the internal structure of the human crystalline lens with age and accommodation.
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Scheimpflug images were made of the unaccommodated and accommodated right eye of 102 subjects ranging in age between 16 and 65 years. In contrast with earlier Scheimpflug studies, the images were corrected for distortion due to the geometry of the Scheimpflug camera and the refraction of the cornea and the lens itself. The different nuclear and cortical layers of the human crystalline lens were determined using densitometry and it was investigated how the thickness of these layers change with age and accommodation. The results show that, with age, the increase in thickness of the cortex is approximately 7 times greater than that of the nucleus. The increase in thickness of the anterior cortex was found to be 1.5 times greater than that of the posterior cortex. It was also found that specific parts of the cortex, known as C1 and C3, showed no significant change in thickness with age, and that the thickening of the cortex is entirely due to the increase in thickness of the C2 zone. With age, the distance between the sulcus (centre of the nucleus) and the cornea does not change. With accommodation, the nucleus becomes thicker, but the thickness of the cortex remains constant. (+info)
Presbyopic phacovitrectomy.
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AIM: To review the results and complications of combined phacoemulsification and vitrectomy in presbyopic patients. METHODS: Retrospective review of 90 consecutive presbyopic patients who underwent phacoemulsification and vitrectomy for primary rhegmatogenous retinal detachment (RRD) (21), redo-RRD (seven), stages 2 and 3 full thickness macular holes (FTMH) (38), stage 4 FTMH (six), idiopathic epiretinal membrane (ERM) (11), proliferative diabetic retinopathy (PDR) (three) and vitreous haemorrhage secondary to branch retinal vein occlusion (BVO) (four). RESULTS: Lens opacity was absent or mild in 84.5% of patients. Reattachment rates for primary RRD and redo-RRD after one procedure were 90.5% and 71.4% and final reattachment rates were 95.2% and 100%, respectively. Macular hole closure rate was 89.5% for stage 2 and 3 FTMH and 83.3% for stage 4 FTMH. There was significant improvement in the median logMAR visual acuity from 1.00 preoperatively to 0.48 postoperatively for the whole cohort (p<0.001, Wilcoxon test). Postoperative complications included fibrinous uveitis (13.3%), iris bombe (2.2%), IOL/iris capture secondary to gas overfill (1.1%), and posterior capsule opacification (51.1%). CONCLUSION: Combined phacoemulsification and vitrectomy is a safe and desirable option in the management of phakic, presbyopic patients with vitreoretinal pathologies that warrant vitreous surgery, even in the absence of significant lens opacity. (+info)